Body weight is controlled in large part by communication between the brain and peripheral metabolic tissues, including white and brown adipose tissue, via the sympathetic nervous system to regulate energy expenditure and lipolysis. Pharmacotherapeutic intervention to reduce adiposity, however, has been relatively unsuccessful. We have identified the neurotrophin-inducible neuronal protein VGF (non-acronymic), and one of its processed C-terminal peptides TLQP-21, as central and peripheral regulators of energy expenditure and lipolysis. TLQP-21 activates the Complement C3a Receptor 1 (C3aR1), an integral component of the innate immune system, and in adipocytes, enhances lipolysis mediated by the beta-adrenergic agonist isoproterenol. Mice with VGF ablated in the adult ventromedial hypothalamus (VMH) and arcuate (ARC) have increased adiposity and decreased energy expenditure, a phenotype that is consistent with a key physiological role for TLQP-21 in the adult CNS, one that is also congruent with many actions of brain-derived neurotrophic factor (BDNF) in the hypothalamus. Utilizing floxed (lox-p flanked) VGF and C3aR1 mouse models together with established transgenic Cre-driver lines and targeted AAV-Cre administration, we will test the hypothesis that in adults, VGF and its peptides, particularly TLQP-21, regulate energy expenditure, lipolysis, and glucose homeostasis via central modulation of sympathetic outflow from the VMH and paraventricular hypothalamus (PVH), which receives extensive VGF-containing projections from ARC/VMH, and provides essential BDNF- and VGF-containing sympathetic outflow pathways to brown adipose tissue (BAT). Two specific aims are proposed. Aim 1 will probe the roles of VGF in the CNS pathways that originate in the PVH and VMH, which can be activated by `designer receptors exclusively activated by designer drugs' (DREADD), and regulate energy expenditure, glucose metabolism and lipolysis via sympathetic outflow from hypothalamus. Aim 2 will define the site(s) of action and function(s) of the pivotal VGF-derived peptide TLQP-21, determining whether its actions in the adult CNS are dependent on C3aR1 that is expressed on neurons, microglia, and/or astrocytes. The complementary research expertise of the PIs will be essential for successful completion of our aims, providing fundamental insight into the mechanisms by which VGF, its peptide TLQP-21, and the TLQP-21 receptor C3aR1, contribute to hypothalamic-sympathetic circuits that control energy and glucose homeostasis, potentially identifying promising new drug targets for the treatment of obesity and diabetes.